Patent GB-693856-A (GB)
Title: Improvements in or relating to optical instruments
IPC: A61B3/08
Filed: 1951-02-13
Abstract: 693,856. Sight-testing apparatus. ELLIOTT OPTICAL CO., Ltd., STONEBRIDGE, E. H., and COMBER, D. J. Feb. 12, 1952 [Feb. 13, 1951], No. 3511/51. Class 97 (i). [Also in Group XL (c)] In an instrument used for orthoptic diagnosis of the kind comprising two pivotably-mounted image-viewing systems, one for each eye of the patient, and each provided with the necessary slide holder and electric lamp illuminating system, manually operable means are provided for controlling each system, and an automatic flashing device is provided having control contacts in each illuminating circuit for obtaining intermittent illumination at a desired frequency together with switching means permitting selection of automatic or manual control for either or both of the viewing systems. As shown in Fig. 1 the automatic flashing device comprises a multivibrator consisting of a double triode valve V with its coupled circuits comprising respectively capacitors Cl, C2 and resistances r3, r5 and r4, r6. Anode resistances r1, r2 are connected respectively to the coils K1, K2 of a double wound relay and are fed with H.T. through a transformer T2, rectifier W and smoothing components L and C. The relay coils K1, K2 respectively control contacts N1 and N2 which alternately engage a central contact N3 connected to one pole of a manual change-over switch S1, S2 and to one side of a secondary l2 of the transformer T forming a source of current for the lamps P1, P2. The multivibrator circuit and contacts N1, N2, N3 may form a separate unit connected by a three-way cable M to the illuminating system of the optical instrument. This system comprises lamps PI, P2 connected to a common return through a variable resistance R3 and to contacts A1, A2 through variable resistances R1, R2, the contacts Al, A2 being operated by manual controls Ql, Q2. Operation of the device is as follows. If automatic flashing is required the switch S1, S2 is opening and the oscillations of the multivibrator cause the contacts N1, N3 and N2, N3 to close alternately whereby the lamps P1, P2 are energized alternately. The intensity of each lamp may be adjusted separately by the resistances R1, R2 and together by adjustment of resistance R3. Variations of the resistances r5, r6 permit variation of the frequency of flashing and the two coupled circuits may be outof-balance to allow one flash to be longer than the other. Either lamp may be switched off by depression of the switch Q1 or Q2 when the other continues to flash, a resistance R4 ensuring no change of intensity of the illuminated lamp when this is done. If simple manual control by the switches Q1, Q2 is required the switch S1, S2 is closed, shortcircuiting the contacts N1, N2, N3 while the secondary l2 continues to supply current to the lamps. A modification of the multivibrator circuit to permit variation of the time each lamp remains illuminated in each flash is shown in Fig. 3 and comprises a valve VI, the control grid Gc of which is connected to the anodes of the valve V by condensers C3 and C4 and to earth through a switch S4. In the anode of this valve is a relay RL controlling a switch RL1 in the circuit between the lamps P1, P2 and the contacts N1, N2, N3. With switch S4 open, switch RL1 is open and on receiving a pulse from the vibrator, the relay RL is energized to close the contact RL1, this taking place after the contact N1 or N2 has been closed by the relays K1 or K2. By choosing the values of the components in this secondary circuit a desired time of flash can be achieved. Closing of the switch S4 cuts out the valve V1. In a modification rectifiers are placed in series with the condensers C3, C4, operation being otherwise the same.